Does this not work for you? According to Dennis P, this should work:
http://forums.ooma.com/viewtopic.php?f= ... ward#p9299
Dennis P wrote:If you choose to stick with the Core service tier, call-waiting will work as long as Call Forward on Busy (CFB) is provisioned on your phone line. You do not need to have ooma Premier to get call-waiting, but you will need to work with your phone company to get CFB added to your line. I will e-mail you offline the phone number that you need to have your calls forwarded to.
Another option is to forward all your calls to ooma (not just when busy). This has the added advantage of you getting Caller-ID (since ooma now sees the incoming call) without paying your phone company for it. The downside of this is that if your Internet connection drops out, then you won't be able to receive calls.
Someone correct my thinking on this?